The present invention relates to microwave integrated circuits, and, more particularly, to microwave integrated circuit package systems. A major objective is to provide a relatively economical, compact, hermetically sealed microwave integrated circuit.
The mass and volume of microwave navigational systems used by aircraft and spacecraft must be minimized. Of the various microwave technologies available, microwave integrated circuit systems require the least mass and volume. A typical microwave integrated circuit includes a microstrip motherboard with active devices mounted on a grounded section thereon. The active devices can include discrete transistors and monolithic microwave integrated circuits (MMICs). A typical MMIC is an integrated circuit fabricated on a gallium arsenide substrate to enhance high frequency operation.
The active components require protection from ambient moisture and other corrosives. Hermetic seals are typically required to achieve the desired protection over the large temperature ranges to which airborne and spaceborne vehicles are subject. Hermetic seals are fused metal-metal, glass-glass, or glass-metal bonds.
While active components can be hermetically sealed individually, a smaller overall package can be achieved by hermetically sealing the entire microwave integrated circuit (MIC) assembly. Typically, a set of MICs are inserted into a metal tub. A cover is then welded onto the tub to effect the hermetic seal.
Tubs have been fabricated of various metals, including stainless steel, Kovar (an alloy of cobalt, iron, and nickel), and aluminum. In the cases of stainless steel and Kovar, the top can be of the same material as the tub. The problem with Kovar and stainless steel is that the materials are relatively heavy and expensive. Aluminum is lighter and more economical. However, components consisting of the most common form of aluminum, the relatively hard industry standard 6061, cannot be welded without danger of cracking. Soft aluminums can be welded to each other, but are not strong enough to provide durable precision machined features such as screw threads. The solution has been to weld a soft aluminum cover to a hard aluminum tub.
A microstrip motherboard typically requires a conductor pattern fabricated on the top of a dielectric substrate. A ground plane is generally required on the bottom of the substrate. However, in some cases, the base of the tub can serve as the ground plane. This latter approach results in a lower profile package. On the other hand, an integral ground plane (e.g., of hard aluminum) provides structural reinforcement for the dielectric, reducing the chances of motherboard breakage. The integral ground plane thus simplifies handling and reduces fabrication costs. The disadvantage is that the resulting package has a higher profile.
Many MIC motherboards use ceramic (e.g., Al.sub.2 O.sub.3 99.6%) as the dielectric material. The thermal coefficient of expansion of ceramic is much lower than for aluminum. Hence, bonding of the ground plane to the ceramic or the ceramic to the tub base is problematic due to differential stresses introduced during temperature swings. Compliant epoxies have become available to reduce the stresses, but the danger of delamination remains.
Recently, temperature-stable dielectric composites of ceramic and polymer have been developed to replace ceramic substrates for MIC purposes. Because they are a brittle thermoset polymer, they lend themselves to thermocompression (or thermosonic) wire bonding. These composites, such as TMM (Temperature-Stable Microwave Material), a brittle thermoset plastic product of Rogers Corporation of Rogers, Conn., have thermal coefficients of expansion that are much closer to aluminum. Hence, problems with delamination are reduced. These materials are relatively inexpensive and more easily fabricated. Ceramic must be sintered, whereas the ceramic/polymer composites and PTFE woven glass can be molded. Once formed, the ceramic/polymer substrates are more easily machined, e.g., by end milling. Furthermore, the ceramic/polymer composites are lighter than the ceramics.
While these several approaches to forming hermetically sealed microwave integrated circuits have been successful, they have generally resulted in packages that are larger, heavier, and more expensive than desired. What is needed is a smaller, lighter, and economical hermetically sealed microwave integrated circuit package.